Switch having virtual shared memory

ABSTRACT

A network of switches having a first switch having a first memory interface and a first expansion port. The network also has an expansion bus having a first expansion bus interface and a second expansion bus interface. The first expansion bus interface is connected to the first expansion port. A second switch has a second memory interface and a second expansion port. The second expansion port is connected to the second expansion bus interface, thereby connecting the first switch to the second switch, wherein the expansion bus allows the first switch to directly access the second memory interface through the second switch and the second switch to directly access the first memory interface through the first switch.

REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application Serial No. 60/237,764 filed on Oct. 3, 2000. The contents of this provisional application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a method and apparatus for high performance switching in local area communications networks such as token ring, ATM, ethernet, fast ethernet, and gigabit ethernet environments, generally known as LANs. In particular, the invention relates to a new switching architecture geared to power efficient and cost sensitive markets, and which can be implemented on a semiconductor substrate such as a silicon chip.

[0004] 2. Description of the Related Art

[0005] As computer performance has increased in recent years, the demands on computer networks has significantly increased; faster computer processors and higher memory capabilities need networks with high bandwidth capabilities to enable high speed transfer of significant amounts of data. The well-known ethernet technology, which is based upon numerous IEEE ethernet standards, is one example of computer networking technology which has been able to be modified and improved to remain a viable computing technology. A more complete discussion of prior art networking systems can be found, for example, in SWITCHED AND FAST ETHERNET, by Breyer and Riley (Ziff-Davis, 1996), and numerous IEEE publications relating to IEEE 802 standards. Based upon the Open Systems Interconnect (OSI) 7-layer reference model, network capabilities have grown through the development of repeaters, bridges, routers, and, more recently, “switches”, which operate with various types of communication media. Thickwire, thinwire, twisted pair, and optical fiber are examples of media which has been used for computer networks. Switches, as they relate to computer networking and to ethernet, are hardware-based devices which control the flow of data packets or cells based upon destination address information which is available in each packet. A properly designed and implemented switch should be capable of receiving a packet and switching the packet to an appropriate output port at what is referred to wirespeed or linespeed, which is the maximum speed capability of the particular network. Basic ethernet wirespeed is up to 10 megabits per second, and Fast Ethernet is up to 100 megabits per second. A gigabit Ethernet is capable of transmitting data over a network at a rate of up to 1,000 megabits per second. As speed has increased, design constraints and design requirements have become more and more complex with respect to following appropriate design and protocol rules and providing a low cost, commercially viable solution.

[0006] Referring to the OSI 7-layer reference model discussed previously, the higher layers typically have more information. Various types of products are available for performing switching-related functions at various levels of the OSI model. Hubs or repeaters operate at layer one, and essentially copy and “broadcast” incoming data to a plurality of spokes of the hub. Layer two switching-related devices are typically referred to as multiport bridges, and are capable of bridging two separate networks. Bridges can build a table of forwarding rules based upon which MAC (media access controller) addresses exist on which ports of the bridge, and pass packets which are destined for an address which is located on an opposite side of the bridge. Bridges typically utilize what is known as the “spanning tree” algorithm to eliminate potential data loops; a data loop is a situation wherein a packet endlessly loops in a network looking for a particular address. The spanning tree algorithm defines a protocol for preventing data loops. Layer three switches, sometimes referred to as routers, can forward packets based upon the destination network address. Layer three switches are capable of learning addresses and maintaining tables thereof which correspond to port mappings. Processing speed for layer three switches can be improved by utilizing specialized high performance hardware, and off loading the host CPU so that instruction decisions do not delay packet forwarding.

SUMMARY OF THE INVENTION

[0007] The invention is directed to a switch having virtual shared memory.

[0008] One embodiment of the invention is a network of switches having a first switch having a first memory interface and a first expansion port. The network also has an expansion bus having a first expansion bus interface and a second expansion bus interface. The first expansion bus interface is connected to the first expansion port. A second switch has a second memory interface and a second expansion port. The second expansion port is connected to the second expansion bus interface, thereby connecting the first switch to the second switch, wherein the expansion bus allows the first switch to directly access the second memory interface through the second switch and the second switch to directly access the first memory interface through the first switch.

[0009] Another embodiment of the invention is a switch for transmitting and receiving data packets. The switch has a memory interface that accesses memory and an expansion port connected to the memory interface. The expansion port is configured to be connected to an expansion bus connected to another switch thereby connecting two switches together allowing for sharing of memory.

[0010] In another embodiment of the invention is a system of network of switches. The system has a first switch having a first memory and a first expansion port and an expansion bus having a first expansion bus end and a second expansion bus end. The first expansion bus end is connected to the first expansion port. The system also has a second switch having a second memory and a second expansion port. The second expansion port is connected to the second expansion bus end, thereby connecting the first switch to the second switch, wherein the expansion bus allows the first switch to directly access the second memory through the second switch and the second switch to directly access the first memory through the first switch.

[0011] Another embodiment of the invention is a method for sharing memory between a first switch and a second switch connected to each other by an expansion bus. The method has the steps of sending a command from a first switch to a second switch that said first switch is about to perform a memory read or write, reading or writing a portion of packet data to local memory of the first switch, and reading or writing another portion of packet data to alternate memory through the second switch using the expansion bus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The objects and features of the invention will be more readily understood with reference to the following description and the attached drawings, wherein:

[0013]FIG. 1 is a general block diagram of elements of the present invention;

[0014]FIG. 2 illustrates the data flow on the CPS channel of a network switch according to the present invention;

[0015]FIG. 3A illustrates a linked list structure of Packet Buffer Memory;

[0016]FIG. 3B illustrates a linked list structure of Packet Buffer Memory with two data packets;

[0017]FIG. 3C illustrates a linked list structure of Packet Buffer Memory after the memory occupied by one data packet is freed;

[0018]FIG. 3D illustrates a linked list structure of Packet Buffer Memory after the memory occupied by another data packet is freed;

[0019]FIG. 4 is a block diagram of two switches having a single shared memory;

[0020]FIG. 5 is a block diagram of two switches having virtual shared memory;

[0021]FIG. 6 is a flow diagram of the steps a switch would take to utilize virtual shared memory.

DETAILED DESCRIPTION OF THE INVENTION:

[0022]FIG. 1 is an example of a block diagram of a switch 100 of the present invention. In this example, switch 100 has 12 ports, 102(1)-102(12), which can be fully integrated IEEE compliant ports. Each of these 12 ports 102(1)-102(12) can be 10BASE-T/100BASE-TX/FX ports each having a physical element (PHY), which can be compliant with IEEE standards. Each of the ports 102(1)-102(12), in one example of the invention, has a port speed that can be forced to a particular configuration or set so that auto-negotiation will determine the optimal speed for each port independently. Each PHY of each of the ports can be connected to a twisted-pair interface using TXOP/N and RXIP/N as transmit and receive protocols, or a fiber interface using FXOP/N and FXIP/N as transmit and receive protocols.

[0023] Each of the ports 102(1)-102(12) has a Media Access Controller (MAC) connected to each corresponding PHY. In one example of the invention, each MAC is a fully compliant IEEE 802.3 MAC. Each MAC can operate at 10 Mbps or 100 Mbps and supports both a full-duplex mode, which allows for data transmission and reception simultaneously, and a half duplex mode, which allows data to be either transmitted or received, but not both at the same time.

[0024] Flow control is provided by each of the MACs. When flow control is implemented, the flow of incoming data packets is managed or controlled to reduce the chances of system resources being exhausted. Although the present embodiment can be a non-blocking, wire speed switch, the memory space available may limit data transmission speeds. For example, during periods of packet flooding (i.e. packet broadcast storms), the available memory can be exhausted rather quickly. In order to enhance the operability of the switch in these types of situations, the present invention can implement two different types of flow control. In full-duplex mode, the present invention can, for example, implement the IEEE 802.3x flow control. In half-duplex mode, the present invention can implement a collision backpressure scheme.

[0025] In one example of the present invention each port has a latency block connected to the MAC. Each of the latency blocks has transmit and receive FIFOs which provide an interface to main packet memory. In this example, if a packet does not successfully transmit from one port to another port within a preset time, the packet will be dropped from the transmit queue.

[0026] In addition to ports 102(1)-102(12), a gigabit interface 104 can be provided on switch 100. Gigabit interface 104 can support a Gigabit Media-Independent Interface (GMII) and a Ten Bit Interface (TBI). The GMII can be fully compliant to IEEE 802.3ab. The GMII can pass data at a rate of 8 bits every 8 ns resulting in a throughput of 2 Gbps including both transmit and receive data. In addition to the GMII, gigabit interface 104 can be configured to be a TBI, which is compatible with many industry standard fiber drivers. Since in some embodiments of the invention the MDIO/MDC interfaces (optical interfaces) are not supported, the gigabit PHY (physical layer) is set into the proper mode by the system designer.

[0027] Gigabit interface 104, like ports 102(1)-102(12), has a PHY, a Gigabit Media Access Controller (GMAC) and a latency block. The GMAC can be a fully compliant IEEE 802.3z MAC operating at 1 Gbps full-duplex only and can connect to a fully compliant GMII or TBI interface through the PHY. In this example, GMAC 108 provides full-duplex flow control mechanisms and a low cost stacking solution for either twisted pair or TBI mode using in-band signaling for management. This low cost stacking solution allows for a ring structure to connect each switch utilizing only one gigabit port.

[0028] A CPU interface 106 is provided on switch 100. In one example of the present invention, CPU interface 106 is an asynchronous 8 or 16 bit I/O device interface. Through this interface a CPU can read internal registers, receive packets, transmit packets and allow for interrupts. CPU interface 106 also allows for a Spanning Tree Protocol to be implemented. In one example of the present invention, a chip select pin is available allowing a single CPU control two switches. In this example an interrupt pin when driven low (i.e., driven to the active state) requiring a pull-up resistor will allow multiple switches to be controlled by a single CPU.

[0029] A switching fabric 108 is also located on switch 100 in one example of the present invention. Switching fabric 108 can allow for full wire speed operation of all ports. A hybrid or virtual shared memory approach can be implemented to minimize bandwidth and memory requirements. This architecture allows for efficient and low latency transfer of packets through the switch and also supports address learning and aging features, VLAN, port trunking and port mirroring.

[0030] Memory interfaces 110, 112 and 114 can be located on switch 100 and allow for the separation of data and control information. Packet buffer memory interface (PBM) 110 handles packet data storage while the transmit queue memory interface (TXM) 112 keeps a list of packets to be transmitted and address table/control memory interface (ATM) 114 handles the address table and header information. Each of these interfaces can use memory such as SSRAM that can be configured in various total amounts and chip sizes.

[0031] PBM 110 is located on switch 100 and can have an external packet buffer memory (not shown) that is used to store the packet during switching operations. In one example of the invention, packet buffer memory is made up of multiple 256 byte buffers. Therefore, one packet may span several buffers within memory. This structure allows for efficient memory usage and minimizes bandwidth overhead. The packet buffer memory can be configurable so that up to 4 Mbytes of memory per chip can be used for a total of 8 Mbytes per 24+2 ports. In this example, efficient memory usage is maintained by allocating 256 byte blocks, which allows storage for up to 32K packets. PBM 110 can be 64 bits wide and can use either a 64 bit wide memory or two 32 bit wide memories and can run at 100 MHz.

[0032] TXM 112 is located on switch 100 and can have an external transmit queue memory (not shown). TXM 112, in this example, maintains 4 priority queues per port and allows for 64K packets per chip and up to 128K packets per system. TXM 112 can run at a speed of up to 100 MHz.

[0033] ATM 114 can be located on switch 100 and can have an external address table/control memory (not shown) used to store the address table and header information corresponding to each 256 byte section of PBM 110. Address table/control memory allows up to 16K unique unicast addresses. The remaining available memory is used for control information. ATM 114, in this example, runs up to 133 MHz.

[0034] Switch 100, in one example of the invention, has a Flow Control Manager 116 that manages the flow of packet data. As each port sends more and more data to the switch, Flow Control Manager 116 can monitor the amount of memory being used by each port 102(1)-102(12) of switch 100 and the switch as a whole. In this example, if one of the ports 102(1)-102(12) or the switch as a whole is using up too much memory as is predetermined by a register setting predefined by the manufacturer or by a user, Flow Control Manager 116 will issue commands over the ATM Bus requesting the port or switch to slow down and may eventually drop packets if necessary.

[0035] In addition to Flow control manager 116, switch 100 also has a Start Point Manager (SPM) 118 connected to Switching Fabric 108, a Forwarding Manager (FM) 120 connected to Switching Fabric 1Address Manager (AM) 122 connected to Switching Fabric 108.

[0036] Start Point Manager (SPM) 118, through Switching Fabric 108 in one example of the present invention, keeps track of which blocks of memory in PBM 110 are being used and which blocks of memory are free.

[0037] Forwarding Manager 120 can, for example, forward packet data through Switching Fabric 108 to appropriate ports for transmission.

[0038] Address Manager (AM) 122 can, through Switching Fabric 108, manage the address table including learning source addresses, assigning headers to packets and keeping track of these addresses. In one example of the invention, AM 122 uses aging to remove addresses from the address table that have not been used for a specified time period or after a sequence of events.

[0039] An expansion port 124 can also be provided on switch 100 to connect two switches together. This will allow for full wire speed operation on twenty-five 100M ports (includes one CPU port) and two gigabit ports. The expansion port 124, in this example, allows for 4.6 Gbps of data to be transmitted between switches.

[0040] An LED controller 126 can also be provided on switch 100. LED controller 126 activates appropriate LEDs to give a user necessary status information.

[0041] Each port of the ports 102(1)-102(12), in one example of the invention, has 4 separate LEDs, which provide per port status information. The LEDs are fully programmable and are made up of port LEDs and other LEDs. Each LED can include a default state for each of the four port LEDs. An example of the default operation of each of the port LEDs are shown below. LED DEFAULT OPERATION 0 Speed Indicator OFF = 10 Mbps or no link ON = 100 Mbps 1 Full/Half/Collision Duplex OFF = The port is in half duplex or no link BLINK = The port is in half duplex and a collision has occurred ON = The port is in full duplex 2 Link/Activity Indicator OFF = Indicates that the port does not have link BLINK = Link is present and receive or transmit activity is occurring on the media ON = Link present without activity 3 Alert Condition OFF = No alert conditions, port is operating normally ON = The port has detected an isolate condition

[0042] In addition to the default operations for the port LEDs, each of the port LEDs can be programmed through registers. These registers can be set up, in one example of the invention, by a CPU. By having programmable registers that control LEDs, full customization of the system architecture can be realized including the programmability of the blink rate.

[0043] Each of the LEDs can have a table, as shown below, associated with the LED, where register bits R_(Ax), R_(Bx) and R_(Cx), can be set to provide a wide range of information. Event ON Condition BLINK Condition OFF Condition Link (L) A₀ = (R_(A0)&L) | !R_(A0) B₀ = (R_(B0)&L) | !R_(B0) C₀ = (R_(C0)&L) | !R_(C0) Isolate (I) A₁ = (R_(A1)&I) | !R_(A1) B₁ = (R_(B1)&I) | !R_(B1) C₁ = (R_(C1)&I) | !R_(C1) Speed (S) A₂ = (R_(A2)&S) | !R_(A2) B₂ = (R_(B2)&S) | !R_(B2) C₂ = (R_(C2)&S) | !R_(C2) Duplex (D) A₃ = (R_(A3)&D) | !R_(A3) B₃ = (R_(B3)&D) | !R_(B3) C₃ = (R_(C3)&D) | !R_(C3) TX/RX Activity A₄ = (R_(A4)&TRA) | !R_(A4) B₄ = (R_(B4)&TRA) | !R_(B4) C₄ = (R_(C4)&TRA) | !R_(C4) (TRA) TX Activity A₅ = (R_(A5)&TA) | !R_(A5) B₅ = (R_(B5)&TA) | !R_(B5) C₅ = (R_(C5)&TA) | !R_(C5) (TA) RX Activity A₆ = (R_(A6)&RA) | !R_(A6) B₅ = (R_(B6)&RA) | !R_(B6) C₆ = (R_(C6)&RA) | !R_(C6) (RA) Auto-Negotiate A₇ = (R_(A7)&N) | !R_(A7) B₇ = (R_(B7)&N) | !R_(B7) C₇ = (R_(C7)&N) | !R_(C7) Active (N) Port Disabled A₈ = (R_(A8)&PD) | !R_(A8) B₈ = (R_(B8)&PD) | !R_(B8) C₈ = (R_(C8)&PD) | !R_(C8) (PD) Collision © A₉ = (R_(A9)&C) | !R_(A9) B₉ = (R_(B9)&C) | !R_(B9) C₉ = (R_(C9)&C) | !R_(C9) Result LED_(ON) = (A₀&A₁&A₂&A₃ LED_(BLINK) = (B₀&B₁&B₂& LED_(OFF) (C₀&C₁&C₂& &A₄&A₅&A₆&A₇&A{overscore (₈)}& B₃&B₄&B₅&B₆&B₇&B₈ C₃&C₄&C₅&C₆&C₇&C₈ A₉){overscore (&(LED)}_(BLI{overscore (NK&LE)}D) _(OFF)) &B₉){overscore (&LE)}D_(OFF) &C₉)

[0044] For example, register bits R_(Ax), R_(Bx) and R_(Cx) can be set to determine when LED_(ON), LED_(BLINK) and LED_(OFF) are activated or deactivated. In addition to the port LEDs, there are additional LEDs which indicate the status of the switch.

[0045] Registers 128 are located on switch 100 in this example of the present invention. Registers 128 are full registers that allow for configuration, status and Remote Monitoring (RMON) management. In this example, Registers 128 are arranged into groups and offsets. There are 32 address groups each of which can contain up to 64 registers.

[0046]FIG. 2 is an illustration of one embodiment of the invention having a PBM Bus, an ATM Bus, and a TXM Bus for communications with other portions of the switch. In this example PBM 110 is connected to the PBM Bus and an external PBM Memory; TXM 112 is connected to the TXM Bus and an external TXM Memory; and ATM 114 is connected to the ATM Bus and an external ATM Memory. Each of the transmit (TX) and receive (RX) portions of ports 102(1)-102(12) are connected to the PBM Bus, ATM Bus and TXM Bus for communications.

[0047] FM 120 is connected to each of the ports 102(1)-102(12) directly and is also connected to the ATM Bus for communications with other portions of the switch. SPM 1122 are also connected to the ATM Bus for communications with other portions of the switch.

[0048] The operation of switch 100 for transmission of a unicast packet (i.e., a packet destined for a single port for output) in one example of the invention is made with reference to FIG. 2 as follows.

[0049] In this example, Switch 100 is initialized following the release of a hardware reset pin. A series of initialization steps will occur including the initialization of external buffer memory and the address table. All ports on the switch will then be disabled and the CPU will enable packet traffic by setting an enable register. As links become available on the ports (ports 102(1)-102(12) and gigabit port 104), an SPT protocol will confirm these ports and the ports will become activated. After the initialization process is concluded normal operation of Switch 100 can begin.

[0050] In this example, once a port has been initialized and activated, a PORT_ACTIVE command is issued by CPU. This indicates that the port is ready to transmit and receive data packets. If for some reason a port goes down or becomes disabled, a PORT_INACTIVE command is issued by the CPU.

[0051] During unicast transmission, a packet from an external source on port 102(1) is received at the receive (RX) PHY of port 102(1).

[0052] In this example, the RX MAC of port 102(1) will not start processing the packet until a Start of Frame Delimiter (SFD) for the packet is detected. When the SFD is detected by the RX MAC portion of port 102(1), the RX MAC will place the packet into a receive (RX) FIFO of the latency block of port 102(1). As the RX FIFO becomes filled, port 102(1) will request an empty receive buffer from the SPM. Once access to the PBM Bus is granted, the RX FIFO Latency block of port 102(1) sends packets received in the RX FIFO to the external PBM Memory through the PBM Bus and PBM 110 until the end of packet is reached.

[0053] The PBM Memory, in this example, is made up of 256 byte buffers. Therefore, one packet may span several buffers within the packet buffer memory if the packet size is greater than 256 bytes. Connections between packet buffers can be maintained through a linked list system in one example of the present invention. A linked list system allows for efficient memory usage and minimized bandwidth overhead and will be explained in further detail with relation to FIG. 3A-FIG. 3D.

[0054] At the same time packets are being sent to the external PBM Memory, the port will also send the source address to Address Manager (AM) 122 and request a filtering table from AM 122.

[0055] If the packet is “good”, as is determined through normal, standard procedures known to those of ordinary skill in the art, such as valid length and IEEE standard packet checking such as a Cyclic Redundancy Check, the port writes the header information to the ATM memory through the ATM Bus and ATM 114. AM 122 sends a RECEP_COMPL command over the ATM Bus signifying that packet reception is complete. Other information is also sent along with the RECEP_COMPL command such as the start address and filtering table which indicates which ports the packet is to be sent out on. For example, a filtering table having a string such as “011111111111” would send the packet to all ports except port 1 and would have a count of 11. The count simply is the number of ports the packet is to be sent, as indicated by the number of “1 ” s.

[0056] Forwarding Manager (FM) 120 is constantly monitoring the ATM Bus to determine if a RECEP_COMPL command has been issued. Once FM 120 has determined that a RECEP_COMPL command has been issued, Forwarding Manager (FM) 120 will use the filtering table to send packets to appropriate ports. It is noted that a packet will not be forwarded if one of the following conditions is met:

[0057] a. The packet contains a CRC error

[0058] b. The PHY signals a receive error

[0059] c. The packet is less than 64 bytes

[0060] d. The packet is greater than 1518 bytes or 1522 bytes depending on register settings

[0061] e. The packet is only forwarded to the receiving port

[0062] The RECEP_COMPL command includes information such as a filter table, a start pointer, priority information and other miscellaneous information. FM 120 will read the filter table to determine if the packet is to be transmitted from one of its ports. If it is determined that the packet is to be transmitted from one of its ports, FM 120 will send the RECEP_COMPL command information directly to the port. In this case, the RECEP_COMPL command information is sent to the TX FIFO of port 102(12).

[0063] If the port is busy, the RECEP_COMPL command information is transferred to TXM Memory through the TXM Bus and TXM 112. The TXM memory contains a queue of packets to be transmitted. TXM Memory is allocated on a per port basis so that if there are ten ports there are ten queues within the TXM Memory allocated to each port. As each of the ports transmitters becomes idle, each port will read the next RECEP_COMPL command information stored in the TXM Memory. The TX FIFO of port 102(12) will receive, as part of the RECEP_COMPL command information, a start pointer which will point to a header in ATM memory across the ATM Bus which in turn points to the location of a packet in the PBM Memory over the PBM Bus. The port will at this point request to load the packet into the transmit (TX) FIFO of port 102(12) and send it out through the MAC and PHY of port 102(12).

[0064] If the port is in half duplex mode, it is possible that a collision could occur and force the packet transmission to start over. If this occurs, the port simply re-requests the bus master and reloads the packet and starts over again. If however, the number of consecutive collisions becomes excessive, the packet will be dropped from the transmission queue.

[0065] Once the port successfully transmits a packet, the port will signal FM 120 that it is done with the current buffer. FM 120 will then decrement a counter which indicates how many more ports must transmit the packet. For example, if a packet is destined to eleven ports for output, the counter, in this example, is set to 11. Each time a packet is successfully transmitted, FM 120 decrements the counter by one. When the counter reaches zero this will indicate that all designated ports have successfully transmitted the packet. FM 120 will then issue a FREE command over the ATM Bus indicating that the memory occupied by the packet in the PBM Memory is no longer needed and can now be freed for other use.

[0066] When SPM 118 detects a FREE command over the ATM Bus, steps are taken to indicate that the space taken by the packet is now free memory.

[0067] Multicast and broadcast packets are handled exactly like unicast packets with the exception that their filter tables will indicate that all or most ports should transmit the packet. This will force the forwarding managers to transmit the packet out on all or most of their ports.

[0068]FIG. 3A is an illustration of a PBM Memory structure in one example of the invention. PBM Memory Structure 300 is a linked list of 256 byte segments 302, 304, 306, 308, 310, 312, 314 and 316. In this example segment 302 is the free_head indicating the beginning of the free memory linked list and segment 316 is the free_tail indicating the last segment of free memory.

[0069] In FIG. 3B two packets have been received and stored in the PBM Memory. Packet 1 occupies segments 302, 306 and 308 and packet 2 occupies segment 304. Segments 310, 312, 314 and 316 are free memory. Segment 310 is the free_head indicating the beginning of free memory and segment 316 is the free_tail indicating the end of free memory.

[0070] In FIG. 3C packet 1 has been fully transmitted and the Forwarding Manager (FM) has issued a FREE command. Since packet 1 is already in a linked list format the SPM can add the memory occupied by packet 1 to the free memory link list. The free_head, segment 310 remains the same. However, the free_tail is changed. This is accomplished by linking segment 316 to the beginning of packet 1, segment 302, and designating the last segment of packet 1, segment 308, as the free_tail. As a result, there is a linked list starting with segment 310 linking to segment 312, segment 312 linking to segment 314, segment 314 linking to segment 316, segment 316 linking to segment 302, segment 302 linking to segment 306 and segment 306 linking to segment 308 where segment 3 free_tail.

[0071]FIG. 3D in this example simply illustrates the PBM Memory after packet 2 has been transmitted successfully and the Forwarding Manager has issued a FREE command over the ATM Bus. The SPM will detect the FREE command and then add the memory space occupied by packet 2 in the PBM Memory to the free memory linked list. In this example segment 308 is linked to the memory occupied by packet 2, segment 304, and segment 304 is identified as the free_tail.

[0072]FIG. 4 is an example of a Centralized Memory 400 being shared between a Switch 410 and a Switch 420 through a Shared Bus 430. Therefore each of the Switches 410 and 420 can access the same memory space allowing for easy data transfer between Switches 410 and 420.

[0073] In order to achieve a bandwidth of 12.8 Gbps in this embodiment, Shared Bus 430 would be a 128 bit bus running at 100 MHz. There are several schemes available to arbitrate bus access, including using a round robin approach that allows each of the Switches 410 and 420 a fixed amount of time on the bus before relinquishing control to the other switch.

[0074]FIG. 5 is an illustration of one embodiment of the present invention. In this example, Switch 510 is connected to a Memory 512 through a Memory Bus 514 and Switch 520 is connected to a Memory 522 through a Memory Bus 524.

[0075] Switch 510 is connected to Switch 520 through an Expansion Bus 530 and a Command Bus 540.

[0076] In one embodiment of the invention each of the buses, Expansion Bus 530, and Memory Buses 514 and 524, are each 64 bit buses. Switch 510 can have an expansion port connecting to Expansion Bus 530 and Switch 520 can have an expansion port connecting to Expansion Bus 530. The combination of Memory 512 and Memory 522 can be a PBM memory for packet data storage, and Memory Buses 514 and 524 can be PBM Memory buses connected to PBMs of Switches 510 and 520, respectively.

[0077] The operation of the configuration illustrated in FIG. 5 is the generally the same as the operation of the configuration illustrated in FIG. 4 in that each chip can access the same memory space and can use the same memory space to communicate and share information. For example, Switch 410 and Switch 420 can each access Memory 400 through Shared Memory Bus 430. Switch 510 can access Memory 512 through Memory Bus 514 and Memory 522 through Expansion Bus 530 and Switch 520. Switch 520 can access Memory 522 through Memory Bus 524 and Memory 512 through Expansion Bus 530 and Switch 510.

[0078] In the example illustrated in FIG. 5, the PBM Memory is split up into two halves, Memory 512 and Memory 522. In order to obtain a bandwidth of 12.8 Gbps as illustrated in FIG. 4, Memory 512 could store an upper half of packet data using a 64 bit bus and Memory 522 could store a lower half of packet data using a separate 64 bit bus. Thus, Memory Bus 514 would be a 64 bit bus, Memory Bus 524 would be a 64 bit bus and Expansion Bus 530 would be a 64 bit bus.

[0079] If Switch 510 reads or writes 128 bits to the PBM Memory, which is made up Memory 512 and 522 in this example, Switch 510 writes the upper 64 bits to local memory, Memory 512 and the lower 64 bits are written to Switch 520 through Expansion Bus 530. At this time, Switch 520 is acting as a proxy. When Switch 520 receives the address and data information from Switch 510, Switch 520 will read or write the lower 64 bits to its local memory, Memory 522. By providing a proxy service, Switch 510 and Switch 520 can read and write 128 bits at a time while maintaining a single memory bus master for each memory and reducing memory bus loading.

[0080] From above, it is evident that there are some differences between the specific operation of the configuration illustrated in FIG. 5 and the operation of the configuration illustrated in FIG. 4. One of the main differences is that the configuration in FIG. 4 has a single Shared Bus 430 and a single Memory 400.

[0081] Therefore, in order to have a memory bandwidth of 12.8 Gbps, Shared Bus 430 would have to be a 128 bit bus running at 100 MHz.

[0082] One advantage of the configuration illustrated in FIG. 5 is that the architecture eliminates the need for both Switches 510 and 520 be electrically connected to a centralized memory along a common bus. If a centralized memory along with a common bus running at 100 MHz were needed, the common bus would have to be 128 bits wide. This will put a high load on the common bus, which can limit the maximum frequency the common bus can handle ultimately affecting the bandwidth the common bus can handle. However, the architecture depicted in FIG. 5 eliminates the need for both Switches 510 and 520 be electrically connected to a centralized memory along a common bus. Instead, Switches 510 and 520 use two 64 bit Memory Buses 514 and 524 through Expansion Bus 530, decreasing the electrical load as compared to the use of a common bus.

[0083] The second advantage of the configuration as illustrated in FIG. 5 is that Switches 510 and 520 do not read and write to the same memory. If each of the of the Switches 510 and 520 wrote to the same common memory each of the Switches 510 and 520 would have to act as a bus master. This will require an arbitration scheme for allowing access to a common bus between Switches 510 and 520 and will also have to allow for recovery time for memory access as the arbitrator switches between the bus masters of Switches 510 and 520. Instead, the configuration as illustrated in FIG. 5 uses a proxy service, where Switch 510 and Switch 520 can read and write 128 bits, using two 64 bit buses, at a time while maintaining a single memory bus master for each memory and reducing memory bus loading.

[0084] Finally, the configuration illustrated in FIG. 5 eliminates the need for memory read cycles to meet setup and hold times for two separate bus masters, since there is only a single bus master and a proxy during read and write cycles to memory.

[0085]FIG. 6 is a flow diagram of the operation of the invention as illustrated in FIG. 5. In this example Switch 510 is going to be writing 128 bits to PBM Memory, which is the combination of Memory 512 and Memory 522.

[0086] In step 610, Switch 510 sends a command to Switch 520 through Command Bus 540. Switch 510 is communicating to Switch 520 that Switch 510 is about to perform a write to PBM memory. Therefore Switch 520 should get ready to act as a proxy to receive data across Expansion Bus 530 for a write to Memory 522.

[0087] In step 620, Switch 510 writes the upper 64 bits to local Memory 512 through Memory Bus 514.

[0088] In step 630, Switch 510 writes the lower 64 bits to Switch 520 through Expansion Bus 530. Switch 520 acts as a proxy and simply writes the lower 64 bits to Memory 522 through Memory Bus 524.

[0089] Thus, the entire write sequence has been completed. It should be understood that a write operation from Switch 520 to PBM memory would operate in the generally the same manner as well as data reads by either Switch 510 or Switch 520.

[0090] The above-discussed configuration of the invention is, in a preferred embodiment, embodied on a semiconductor substrate, such as silicon, with appropriate semiconductor manufacturing techniques and based upon a circuit layout which would, based upon the embodiments discussed above, be apparent to those skilled in the art. A person of skill in the art with respect to semiconductor design and manufacturing would be able to implement the various modules, interfaces, and tables, buffers, etc. of the present invention onto a single semiconductor substrate, based upon the architectural description discussed above. It would also be within the scope of the invention to implement the disclosed elements of the invention in discrete electronic components, thereby taking advantage of the functional aspects of the invention without maximizing the advantages through the use of a single semiconductor substrate.

[0091] Although the invention has been described based upon these preferred embodiments, it would be apparent to those of skilled in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention. In order to determine the metes and bounds of the invention, therefore, reference should be made to the appended claims. 

We claim:
 1. A network of switches comprising: a first switch having a first memory interface and a first expansion port; an expansion bus having a first expansion bus interface and a second expansion bus interface, said first expansion bus interface connected to said first expansion port; and a second switch having a second memory interface and a second expansion port, said second expansion port connected to said second expansion bus interface, thereby connecting said first switch to said second switch, wherein said expansion bus allows said first switch to directly access said second memory interface through said second switch and said second switch to directly access said first memory interface through said first switch.
 2. The network of switches as recited in claim 1, wherein said first expansion port further comprises a first proxy component that enables data packets to be read from said first memory and written to said first memory by said second switch through said expansion bus, and wherein said second expansion port further comprises a second proxy component that enables data packets to be read from said second memory and written to said second memory by said first switch through said expansion bus.
 3. The network of switch as recited in claim 1 wherein said first memory interface is configured to be connected to a first external memory and said second memory interface is configures to be connected to a second external memory.
 4. The network of switches as recited in claim 1 further comprising a command bus connected between said first switch and said second switch allowing commands to be communicated between said first switch and said second switch.
 5. A switch for transmitting and receiving data packets comprising: a memory interface that accesses memory; and an expansion port connected to said memory interface, wherein said expansion port is configured to be connected to an expansion bus connected to another switch thereby connecting two switches together allowing for sharing of memory.
 6. The switch as recited in claim 5 wherein said expansion port further comprises a proxy component that when activated allows data packets to be read from said memory and written to said memory from another switch through said expansion port.
 7. The switch as recited in claim 5 wherein said memory interface is configured to access external memory.
 8. The switch as recited in claim 5 further comprising a command bus interface configured to be connected to another switch allowing commands to be communicated between switches.
 9. A system of network of switches, said system comprising: a first switch having a first memory and a first expansion port; an expansion bus having a first expansion bus end and a second expansion bus end, said first expansion bus end connected to said first expansion port; and a second switch having a second memory and a second expansion port, said second expansion port connected to said second expansion bus end, thereby connecting said first switch to said second switch, wherein said expansion bus allows said first switch to directly access said second memory through said second switch and said second switch to directly access said first memory through said first switch.
 10. The system of network switches as recited in claim 9, wherein said first expansion port further comprises a first proxy component that when activated allows data packets to be directly read from said first memory and directly written to said first memory by said second switch through said expansion bus, and wherein said second expansion port further comprises a second proxy component that when activated allows data packets to be directly read from said second memory and directly written to said second memory by said first switch through said expansion bus.
 11. The system of network switches as recited in claim 9, wherein said first memory is external memory and said second memory is external memory.
 12. The system of network switches as recited in claim 9 further comprising a command bus connected between said first switch and said second switch allowing commands to be communicated between said first switch and said second switch.
 13. A method for sharing memory between a first switch and a second switch connected to each other by an expansion bus comprising the steps of: sending a command from a first switch to a second switch that said first switch is about to perform a memory read or write; reading or writing a portion of packet data to local memory of said first switch; and reading or writing another portion of packet data to alternate memory through said second switch using said expansion bus.
 14. The method as recited in claim 13, wherein said step of sending a command further comprises configuring said second switch to be a proxy allowing said packet data to be read from said second memory or written to said second memory by said first switch through said expansion bus.
 15. The method as recited in claim 13, wherein said step of sending a command comprises the step of sending said command across a command bus connected between said first switch and said second switch allowing commands to be communicated between said first switch and said second switch. 